fix(lite): surface sync failures instead of a frozen "Syncing 99.9%"; don't report a lost server as "Synced"

Lite-parity audit, sync-progress semantics (state/presentation only — no send/sync backend call changed):
- A scan failure set scan->failed + lite_error, but no UI read lite_error, so the user saw "Syncing 99.9%"
  forever. The status bar now shows the error text on a lite scan/server failure.
- After the initial catch-up, a server loss on the cheap probe path (ProbeFailed) left the last "Synced"
  model on screen. The worker now re-publishes that model demoted (not complete, reason "server unreachable").
- refreshModelImpl's height-read "synced" override no longer fires while syncstatus says the backend is
  still scanning or is indeterminate, and a server-bound read failure in a full cycle demotes to not-synced.
- The syncstatus parser no longer treats a missing/unrecognised "syncing" field as fully synced: it reports
  an explicit "indeterminate" (not complete). Explicit "false" stays caught-up; the fake backend's
  syncing:true 1000/1000 behaviour is unchanged.

Tests: parser cases for missing/odd/idle "syncing" (testLiteSyncStatus*).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1595,6 +1595,7 @@ void I18n::loadBuiltinEnglish()
         strings_["sb_mining_hs"] = "%.1f H/s";
         strings_["sb_syncing_eta"] = "Syncing %.1f%% (%d left, %.0f blk/s, ~%s)";
         strings_["sb_syncing_basic"] = "Syncing %.1f%% (%d left)";
+        strings_["sb_lite_sync_error"] = "Sync error: %.80s";
         strings_["sb_rescanning_pct"] = "Rescanning %.0f%%";
         strings_["sb_rescanning"] = "Rescanning";
         strings_["sb_building_witnesses_pct"] = "Rebuilding witnesses %.0f%%";